Several aromatic carboxylic acids were identified mass-spectroscopically as benzoic, 4-methoxybenzoic, 3, 4-dimethoxybenzoic, phenylacetic, 4-methoxyphenylacetic, and 3, 4-dimethoxyphenylacetic acids in a ca. 6,500 years old organic sediment dated C-14 determination with accelerator mass spectrometry taken from Taihu Lake on Shanghai Delta in China, and there were not found any ligninrelated phenolic acids. The present study is focused on the organic matter in the sediments whether the aromatic acids identified in the organic sediment are transgenic from L-phenolic acids or the organics is autochtonous in the lake by using both determinations of stable carbon isotope ratio and hydrocarbons ratio of n-CィイD215ィエD2 + n-CィイD217ィエD2 + n-CィイD219ィエD2 to n-CィイD227ィエD2 + n-CィイD229ィエD2 + n-CィイD231ィエD2.By determining hydrocarbons (CィイD215ィエD2 to CィイD231ィエD2), the organic matter in the organic-rich layer of the sediment fore seems to have originated from the terrestrial production : the ratio of n-CィイD215ィエD2 + n-CィイD217ィエD2 + n-CィイD219ィエD2 to n-CィイD227ィエD2 + n-CィイD229ィエD2 + n-CィイD231ィエD2 was 15, suggesting a considerable contribution from terrestrial plants. Among the stable isotope findings was the following : organic matter in the organic-rich layer of the sediment core seems to have originated from the primary production of a forest ecosystem unlike that in organic-poor layers : the delta value for carbon (δィイD113ィエD1C = -29.2) indicate a relatively simple cycling : at the lowest layer of the sediment profile examined, the delta value for caron (δィイD113ィエD1C = -13) were higher than all the upper layers, suggensting a considerable contribution from C4 plants.Futher study will be needed to clarify transgenic mechanism to the aromatic acids from L-phenolic acids furing diagenesis in an aquatic sedimentary environment.